I may catch some shit for these, but
I like Pantera's cover of "Planet Caravan" or Sabbath's version, I know, heresy.
Van Halen's "You Really Got Me" over The Kinks
RATM's "How Could I Just Kill A Man" over Cyprus Hill
Foo Fighter's "Down In The Park" and Fear Factory's "Cars" I was just never a fan of Gary Newman's voice.
Ok, here's the big one, I love Blue Oyster Cult, but, Metallica truly owned Astronomy.
And I'm sorry, but seriously, GnR's version of Sympathy? I was just cringing at it the other day when they played it on the radio, even Slash is on the record saying "This is the sound of the band ending."